Technical Abstract:
The seed coat is a vital tissue for directing the flow of photosynthate from source leaves to the embryo and cotyledons during seed development. By forming a sucrose gradient, the seed coat promotes transport of sugars from source leaves to seeds, thereby establishing sink strength. Understanding the mechanistic basis of seed coat-mediated translocation is of interest, especially during periods of abiotic stress conditions. This project aims to characterize gene expression changes in the seed coat that result from abiotic stress-induced reductions in photosynthesis. Soybean plants were exposed to elevated temperature (+3.5 °C), drought and elevated ozone concentrations (100 ppb) during the 2011 and 2012 growing seasons at that SoyFACE facility (http://www.igb.illinois.edu/soyface/). All three abiotic stress treatments reduced rates of photosynthetic carbon assimilation, which resulted in decreased leaf total nonstructural carbohydrate content. However, there was no significant difference in the carbohydrate content in the petioles of plants exposed to heat, drought or ozone, and a strong gradient in sucrose was apparent from the seed coat to the cotyledon, indicating that sink strength was maintained in the abiotic stress treatments. Analysis of the seed coat transcriptome showed high expression levels of genes related to cell wall invertase activity, sucrose transport, and D-type cyclins, which integrate nutrient signals in cell cycle responses in developing seeds. Furthermore, genes related to drought tolerance and sucrose catabolism were differentially expressed in the seed coat under stress. Finally, 14 genes were significantly differentially expressed across all abiotic stress treatments, including genes encoding heat shock proteins, ABC-type transporter family proteins, integrase-type DNA-binding superfamily proteins, and homeobox-leucine zipper family proteins. These genes are of interest for future functional analysis. Overall this study showed that the seed coat maintains sink strength, even under abiotic stress conditions, such that individual seed size was not altered by heat, drought or ozone. Results from this study provide a significant contribution to the current body of work on understanding soybean yield decreases under abiotic stress conditions, which is crucial for ensuring future food security under future climate change scenarios.